Project summary This is a competing renewal application for a grant that has provided 10 years of support for studies of the interaction between host immunity, evolution and epidemiology of Streptococcus pneumoniae (pneumococcus) using approaches including molecular genetics and immunology, molecular and classical epidemiology, and mathematical modeling. Pneumococcus serves as a model system for understanding how natural and vaccine-induced immunity create selection pressure on pathogens to diversify antigenic molecules or otherwise escape host immune responses. This application focuses on a genome-wide understanding of antigenic variability in protein antigens, i.e. virtually all important antigens other than the polysaccharide capsule. The first aim uses age- structured, transmission-dynamic modeling of selection pressures from allele-specific immunity within a host population to assess what patterns one would expect to see in genes encoding antigens targeted by such immune responses. These include patterns of sequence polymorphism, presence/absence polymorphism, and differences in bacterial populations in older (more immune) vs. younger (less immune) hosts. This aim will formalize the expectations of what sorts of patterns are expected in pneumococcal populations (Aim 2). The output is a proposed signature of antigenic diversifying selection that is expected to be observed in bacterial genomes sampled from human hosts. In Aim 2, we will validate and apply the model using novel population genomic data. We will perform whole genome sequencing of 800 pneumococcal isolates from a defined population (Navajo and Apache in the US Southwest) and apply the measures defined in Aim 1 to assess the evidence that each gene in the genome is under diversifying selection, and test these predictions against mechanistically defined lists of antigens. Aim 3 will test in vitro whether allelic variation of putative antigens (two pre- defined proteins, PspC and RrgB, and 2-3 others identified in Aim 2) permits escape from two functions of antibody action: opsonization and blocking of attachment This work will bring new precision to the study of how host immunity leads to diversification of pathogens, developing concepts with application beyond the pneumococcus.